The Multicenter Study of Hydroxyurea (MSH) is a phase III clinical trial that attempts to determine whether hydroxyurea can reduce sickle cell pain crisis attacks by at least 50 percent. The study has a double-blind, placebo-controlled design. All 299 patients, enrolled nationwide, are adults with sickle cell anemia (Hb SS); Howard University has contributed patients to the study. The clinical trial involves the daily administration of hydroxyurea (15-30mg/kg day) or placebo capsules, which are dispensed during outpatient visits every two weeks. Data on vaso- occlusive crises will be collected by the clinical centers and verified by a "blinded" central committee that will define whether each medical contact was, or was not, a vaso-occlusive event according to standardized criteria. Enrollment for this trial started in early 1992, and data collection to test the main hypothesis is scheduled to close by the end of April 1995. At that time, each patient will have been on the maximum tolerated hydroxyurea dose for at least one year. The study, however, will continue for another six months (until September 1, 1995) for purposes of data editing and clean-up, and for analysis of secondary endpoints. In addition, it is quite possible that the participating clinical centers will be funded for follow-up of study participants in an attempt to determine if there are long-term adverse effects of hydroxyurea administration. If this clinical trial shows that hydroxyurea ameliorates the vaso-occlusive severity of sickle cell anemia (Hb SS), there are plans for testing it in other patients, such as those with sickle-cell hemoglobin-C disease (Hb SC), who are not eligible for the current protocol.